1. Field of the Invention
The present invention is directed to an arrangement for data transmission which utilizes a telephone network connection between modems for data terminal equipment that is connected to a telephone switching center via telephone subscriber lines.
2. Description of the Related Art
In a telephone network that is utilized for data communications, modems (which include modulators and demodulators) provide a data transmission means that are arranged as a link for adapting between telephone voice paths and data terminal equipment and that serve the purpose of converting the digital signals supplied by the data terminal equipment into line signals which are advantageous for data transmission and which correspond to the electrical conditions for analog telephone connections or which undertake a back-conversion of such line signals into digital signals.
Different modulation methods, such as frequency modulation, phase-difference modulation and quadrature amplitude modulation, are applied in this context (see, for example, Handworterbuch des elektrischen Fernmeldewesens, published on commission of the Bundesministerium fur das Post- und Fernmeldewesen).
There are a multitude of different modem protocols for modem connections including, for example, those specified by the ITU-T (formerly CCITT) in what is referred to as the V-series.
FIG. 1 of the drawings shows how data transmission was previously sequenced over the telephone network.
Data signals which are derived from data terminal equipment, for example a personal computer PC1, are converted into line signals in a modulator MOD1 and are transmitted to a telephone switching center LE1 via a subscriber line TL1, which mainly serves for transmitting voice signals from and to telephone terminal equipment. In a subscriber line circuit TSCH1 to which the subscriber line TL1 is connected, the line signals experience the same handling as voice signals, namely a band limitation to 300 through 3400 Hz, an analog-to-digital conversion, a compression according to the A-law or the .mu.-law, and are ultimately transmitted via a PCM link PCM of the telephone network SPN, to which the switching center LE1 also belongs, to a destination switching center LE2 of the telephone network to which the other data terminal equipment that is in the form of the personal computer PC2 for the connection under consideration is connected. In the subscriber line circuit TSCH2 of the destination switching center LE2, the transmitted data experience a corresponding back-conversion, namely an A-law or .mu.-law expansion and a digital-to-analog conversion, before they are transmitted via the telephone subscriber line TL2 to the modem MOD2, in order to proceed therefrom to the data terminal equipment PC2 after conversion into digital signals.
Except for the data terminal equipment, the system parts mentioned in conjunction with the described data signal transmission, which mainly serve for voice signal transmission between telephone terminal equipment such as the illustrated terminal equipment Tel1 and Tel2, are accordingly optimized for the voice signal transmission and therefore have a limiting effect for the data transmission in view of the transmission rate and the transmission bandwidth. Highly complex modulation methods for the digital data are utilized in order to achieve an enhancement of the obtainable data rates, this, of course, resulting in correspondingly increased outlay.
Given the above-described, known way of sequencing data transmissions using the telephone network, there is a direct communication of the subscribers--as in voice signal connections--with one another. In this case, the subscribers are the modems. This requires that the subscribers must "understand one another", i.e. that the same modem protocol must be employed at both modems at the two terminal equipment sides. A further demand arises when modem protocols are reformulated since these new protocols must also cover previous modem protocols so that modems which are already installed can continue to be employed.